Work in this group has focused on the molecular and cellular mechanisms that govern the intracellular localization and trafficking of newly synthesized proteins in the central vacuolar system. Specific areas of interest have included the relationship between protein assembly and transport from the endoplasmic reticulum (ER) and the mechanisms of protein localization to the trans-Golgi network (TGN). Studies of protein assembly in the ER were done using the multisubunit class II molecules of the major histocompatibility complex (MHC) as a model system. Class II MHC molecules are composed of two polymorphic chains (alpha and beta), that associate in the ER with a third non-polymorphic chain known as the invariant chain (I). Our studies have shown that various incompletely assembled forms of class II MHC molecules are largely retained in the ER, where they bind non-covalently to the immunoglobulin heavy chain binding protein, BiP. In addition, the incomplete complexes have been found to form large homotypic aggregates. On the basis of these observations, we have proposed that interaction with ER resident proteins such as BiP and aggregation are major determining factors in the retention of incompletely assembled complexes in the ER. We have also extended our previous studies on protein localization to the TGN to the dibasic endopeptidase, furin. The cytoplasmic domain of furin has been found to encode information specifying localization to the Golgi system. Together with our earlier studies on the TGN protein, TGN38, these findings strongly suggest the existence of a general mechanism for protein localization to the Golgi system that relies on the recognition of cytoplasmic signals. In order to identify genes involved in this mechanism, we have used a genetic screen for yeast mutants defective in the localization of the yeast homologue of furin, kex2p. This approach has yielded several mutant clones that are currently being further characterized genetically and biochemically.